April 4, 2011

I've used the DS1307 Real-Time Clock (RTC) for a few projects in the past, but I'm currently working on several datalogger projects that use both RTC and SD-card. The problem is that the SD card won't survive 5V, and the DS1307 won't work at 3.3V. That particular clock chip needs a minimum of 4.5V according to both the datasheet and some inadvertent "experimental verification".

Rex Belli (One of my students and an all-around bright guy, contact me if you're hiring a summer intern) pointed me towards the DS3231 as a possible replacement. It has several advantages over the DS1307:

It runs fine on either 3.3V or 5V.

It has a built-in oscillator: no external crystal required.

It has two built-in alarms that can drive an interrupt pin, so if you just need a periodic interrupt signal this chip can in many cases do the job without a microcontroller.

It' rated to 2 minutes per year (max) drift. (My best DS1307 clock drifts about 2 minutes per month!)

One disadvantage is that the DS3231 doesn't have the eprom storage that the DS1307 has, but you can cheat and store seven bytes in the alarm registers if you had to.

I wrote an Arduino library for it, so if you want to use this clock chip with that microcontroller it makes things a bit easier.

Here's the library: DS3231.zip
The header file (DS3231.h) is extensively commented, and there are several example sketches included as well. Enjoy! If you use the library for anything interesting, send me an email. I'd be happy to hear what's been done with it.